A generalized strategy has been developed by a team from the Nanophotonics and NanoBio Interfaces Groups at Argonne’s Center for Nanoscale Materials (CNM) for the synthesis of mesoporous colloidal superparticles (CSPs) made of nanocrystals of platinum-group metals (e.g., Pt, Pd, Rh, and Ir) through self-limited growth of metal nanocrystals (or precursors) and silver halide in individual colloidal particles. The silver halide components can be selectively dissolved to expose the surfaces of individual metal nanocrystals in the CSPs. By controlling the reaction conditions, the size and composition of the CSPs can be easily tuned. For example, Pt CSPs with sizes ranging from 20 to 150 nm were synthesized by varying the ratio of a Pt precursor and the sacrificial Ag nanoparticle precursor. This strategy can be extended to the synthesis of composite CSPs from coprecipitation of two types of components, one of which can be selectively removed at mild conditions that do not significantly influence the properties of the other component to form mesoporous CSPs. The as-prepared mesoporous CSPs are free of organic ligands, resulting in clean surfaces with more accessible active sites for applications such as catalysis. The high-density negative charges due to the adsorption of halide ions enable excellent stability of the mesoporous CSPs in polar solvents (e.g., water and ethanol) that is beneficial for applications and post processing. Due to these advantages, the synthesized mesoporous Pt CSPs exhibit significantly improved catalytic activity and recyclability in catalyzing solution-phase electron-transfer reactions in comparison with the well-dispersed small Pt nanocrystals. As an emerging class of novel nanostructures, controlled synthesis of CSPs represents an important research area because of their unique properties originated from the coupling of individual nanocrystals in CSPs other than the intrinsic physical/chemical properties from the constituent nanocrystals. Assembly of the highly uniform CSPs into more complex, hierarchically-ordered materials may enable new properties and the discovery of new applications.
The CSPs catalyzed an electron-transfer reaction involving in the reduction of hexacyanoferrate (III) (Fe(CN)63-) with thiosulfate ions (S2O32-). This reaction has been used widely metal nanostructures because the reaction does not occur without a catalyst and the reaction can be monitored easily. 88% of Fe(CN)63- ions were reduced when the reaction lasted 35 minutes. In contrast, using same amount of small Pt nanocrystals (with sizes of 3 nm) only 40% of Fe(CN)63- ions to Fe(CN)64- ions were converted even when the reaction lasted over 100 minutes. Furthermore, CSPs can be easily recovered via centrifugation while the smaller Pt nanocrystals are extremely difficult (or even impossible) to recover from the reaction solution. These distinct differences clearly demonstrate the promise of the porous platinum-group metal CSPs in high-performance catalysis.
“Mesoporous Colloidal Superparticles of Platinum-Group Nanocrystals with Surfactant-Free Surfaces and Enhanced Heterogeneous Catalysis,” Yongxing Hu, Yuzi Liu, and Yugang Sun, Advanced Functional Materials, 25, 1638 (2015)